Mullen made the comments in Lithuania's capital Vilnius at a news conference with Valdas Adamkus, the country's president.
"I certainly understand the reactions in places like Ukraine or the Baltics after what had happened in Georgia," Mullen said.
"And in part why I am here is... to reassure your countries, that the US is very committed to those obligations."
Mullen, who visited Latvia on Tuesday, said the US had "made that [reassurance to Baltic states] very clear" to General Nikolai Makarov, head of the Russian general staff, during a meeting in Helsinki this week.
The admiral said he saw a need for more joint military exercises with Baltic states, both in the Nato framework and bilaterally with Washington.
The US has just taken over the rotating air police mission over the Baltic states, which have no fighter planes of their own.